$=1-(\frac {1}{20}+\frac {1}{30}+\frac {1}{42}+\frac {1}{56}+\frac {1}{72}+\frac {1}{90}+\frac {1}{110}+\frac {1}{132})$
$=1-(\frac {1}{4}-\frac {1}{5}+\frac {1}{5}-\frac {1}{6}+···+\frac {1}{11}-\frac {1}{12})$
$=1- (\frac {1}{4}-\frac {1}{12})$
$=1-\frac {1}{6}$
$=\frac {5}{6}$
